Dr. Peter William de Oliveira

In this paper, the synthesis route and growth mechanism of ZnO nanowhiskers and multipods are investigated. A simple chemical deposition route is used to synthesize tin doped zinc oxide (TZO) nanostructures using a horizontal quartz furnace. To study the effect of tin doping on the morphology of ZnO thin film, nanostructures with different morphologies and sizes were grown on silicon wafers and ITO substrates using thermal evaporation of a mixture of zinc, tin, and graphite powders at 1000 °C in a pure oxygen atmosphere. Structural studies of the synthesized ZnO nanostructures were done by means of XRD, SEM, and HRTEM. An XRD graph confirmed the formation of a pure ZnO phase. The SEM and HRTEM images exhibited nanowhiskers with dimensions of an average width of 100 nm and a few micrometers in length. The photoluminescent properties of these structures were also investigated where it shows strong blue-green emission at room temperature for the samples doped with tin.

Stable crystalline aluminum doped zinc oxide (AZO) nanopowders were synthesized using hydrothermal treatment processing. Three different aluminum precursors have been used. The Al-precursors were found to affect the morphology of the obtained nanopowders. AZO nanoparticles based on zinc acetate and aluminum nitrate have been prepared with different Al/Zn molar ratios. XRD investigations revealed that all the obtained powders have single phase zincite structure with purity of about 99%. The effect of aluminum doping ratio in AZO nanoparticles (based on Al-nitrate precursor) on structure, phase composition, and particle size has been investigated. The incorporation of Al in ZnO was confirmed by UV-Vis spectroscopy revealing a blue shift due to Burstein-Moss effect.

Anti-reflective (AR) single layer of silica-titania (SiO2-TiO2) coatings were obtained from sols containing pyromellitic dianhydride (PMDA) derivatives and Ti and Si precursors on glass substrate by dip-coating method. The coatings showed very high optical quality and the transmission was improved to up to 98.5%. Furthermore, the coatings also presented good mechanical stability.

In the present study, cadmium sulfide (CdS) thin films were deposited on different substrates [soda glass, fluoride doped tin oxide, and tin doped indium oxide (ITO) coated glass] by a hot plate method. To control the thickness and the reproducibility of the sample production, the thin films were coated at different temperatures and deposition times. The CdS thin films were heated at 400 °C in air and forming gas (FG) atmosphere to investigate the effect of the annealing temperatures. The thickness of the samples, measured by ellipsometry, could be controlled by the deposition time and temperature of the hot plate. The phase formation and structural properties of CdS thin films were studied by X-ray diffraction and scanning electron microscopy, whereas the optical properties were obtained by UV–vis spectroscopy. A hexagonal crystal structure was observed for CdS thin films and the crystallinity improved upon annealing. The structural and optical properties of CdS thin films were also enhanced by annealing at 400 °C in FG atmosphere (95 % N2, 5 % H2). The optical band gap was changed from 2.25 to 2.40 eV at different annealing temperatures and gas atmospheres. A higher electrical conductivity, for the sample annealed at FG, was noticed. The samples deposited on ITO and annealed in FG atmosphere showed the best structural and electrical properties compared to the other samples. CdS thin films can be widely used for application as a buffer layer for copper–indium–gallium–selenide solar cells.

The effect of the addition of trioxadecanoic acid (TODS) on the performance of Dye sensitized solar cells (DSSCs) was studied. An increase of the overall light to current conversion efficiency was observed, due to a significant increase of the photocurrent density. The increase of the photocurrent density referred to the more homogeneous porous structure and the relevant increase of the surface area, which allowed higher amount of dye loading. This resulted in a positive impact on the enhancement of the photo current density. The impedance spectroscopy showed that the addition of TODS resulted in a decrease of the charge transfer resistance of the cell, which explained by increasing the charge regeneration.

Crystalline indium doped zinc oxide (IZO) nanopowders were synthesized using hydrothermal treatment processing. Increasing the doping ratio of indium in the zinc oxide significantly influences the phase structure and shape of the nanopowders resulting in nanorod to nanoparticulate morphologies. As the doping profile increases, the crystallite size decreases, the band gap energy blue shifts and the specific surface area increases (measured by BET method). Additionally Raman spectroscopy exhibited shifts of several peaks, as well as revealed new peaks, confirming the substitution of indium ions within the zinc oxide lattice sites. An IZO suspension made of IZO nanoparticles (In/Zn = 3 atm.%) with a zeta potential greater than 30 mV at pH = 6 was successfully spin-coated on glass substrates for to make transparent conductive coatings evincing sheet resistances as low as 35 kΩ□ (ρ = 4.9 × 10−3 Ω m,) with transmission in the visible range as high as 90 %.

In this paper, we report the synthesis of one-dimensional zinc oxide (ZnO) nanostructures and the impact of their morphology on oxygen gas sensing properties. The nanostructures were synthesised via chemical vapour deposition using direct oxidation in an electrical furnace. Structural characterisation of the samples was performed with a field emission scanning electron microscope (SEM) and X-ray diffraction (XRD) methods. The SEM images revealed the formation of different sized nanowires, nanorods and nanoflower structures, and the XRD pattern showed hexagonal structures, without any impurities. The gas sensing properties of samples grown on silicon and alumina substrates were measured in different conditions. The samples grown on the alumina substrate showed better gas sensing properties than those grown on the silicon. To determine the optimal sensitivity, the oxygen gas sensing properties of the ZnO nanostructures were measured at different temperatures and gas flows. These nanostructural gas sensors showed high sensitivity at temperatures close to ambient. The effect of the morphology of ZnO nanostructures on their oxygen sensing properties was compared. Between the different synthesised nanostructures, ZnO nanowires exhibited the highest gas sensitivity.

In this paper, we studied the effects of the aluminium dopant concentration on the optical and electrical properties of aluminium doped zinc oxide (AZO) thin films grown on soda-glass substrates by a simple chemical method. The amount of aluminium in the compound was varied from 0 to 5 atomic percent (at.%), and the typical thickness of the films produced was about 300 nm. The thin films were characterized by scanning electron microscopy and X-ray diffraction to investigate the morphology and crystallinity of the samples. The optical properties of the thin films were studied by UV-Vis spectroscopy to determinate absorption, transmittance, and the diffuse reflectance. In addition, the photoluminescence properties of the thin films, excited with a 320 nm UV laser beam, were investigated. The effects of the aluminium concentration on these optical properties are discussed. The films with 2 and 5 % doping had excellent optical transmittance (~85-90 %) in the 400-1100 nm wavelength range. The photoluminescence spectra of the AZO films revealed UV near band edge emission peaks in the 378-401 nm range and an oxygen-vacancy related peak around 471 nm. The addition of aluminium changed the band gap of zinc oxide from 3.29 to 3.41 eV, and the appearance of a new level was observed in the band gap at the higher aluminium doping concentrations. The AZO thin films showed good conductivity (in the order of 10-2 Ω cm) which allows their use as transparent electrodes. Moreover, the AZO thin films were stable in open air for 30 days.

Using direct oxidation of Zn powder as a source material, zinc oxide comb and saw structures were fabricated by vapor phase transport and condensation. X-ray diffraction pattern showed that the samples had a wurtzite crystal structure. Different structures such as comb-like nano- and microstructures and saw-like structures have been studied by scanning electron microscopy. ZnO comb-like nanostructures with an average size of 15–20 nm and 50 nm were observed, as well as comb-like and saw-like microstructures. Photoluminescence (PL) spectra of these ZnO samples at room temperature were discussed. PL spectra showed strong UV and green emissions that indicate their possible application in optoelectronic devices. Also, the I–V behavior of a single nanowire was investigated. Approximately linear current versus voltage was observed for this nanowire, which showed good ohmic contacts to structure.

In this work, Cu(In,Ga)Se2 (CIGS) nanoparticles were synthesized using a wet chemical method. The method is based on a non-vacuum thermal process that does not use selenization. The effects of temperature, source materials, and growth conditions on the phase and particle size were investigated. X-ray diffraction results confirm the formation of a tetragonal CIGS structure as the main phase with the purity more than 99% obtained by energy-dispersive X-ray spectroscopy (EDX). The morphology and size of the samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Using these methods, 20-80 nm particles were obtained. Through measurements of the absorption spectra of CIGS nanoparticles, the band gap of the synthesized material was determined to be about 1.44 eV, which corresponds to an acceptable wavelength region for absorber layers in solar cells.